Recently a new type of latex-based caulk has appeared on the market. When applied and allowed to dry, the caulk beads are transparent rather than white or pigmented. To be transparent, the caulk cannot contain any of the customary fillers. Therefore, to achieve the requisite viscosity of caulks, the latex is formulated with a thickener. Fumed silica is a common thickener as are cellulosics, polyacrylic acids and polyurethane inverse emulsions. Frequently, fumed silica is used in conjunction with one of the other thickeners as it provides excellent adhesion to glass and tile. However, the incorporation of fumed silica imparts a haze to the dried film so that the dried bead is no longer truly transparent but rather only translucent.
To date, this problem has been ignored by suppliers to the market. Either the suppliers call their product a translucent caulk or they mislead customers and call it transparent. Other manufacturers omit the fumed silica and sacrifice the adhesion, they target the lower end uses.
U.S. Pat. No. 4.626,567 discloses an acrylic copolymer latex sealant composition, which comprises an acrylic copolymer having a glass transition temperature not exceeding about 0.degree. C., a hydrolyzed organoaminosilane, and anionic, nonionic or amphoteric surfactant, water and at least about 0.5% by weight silica, based on the total amount of the composition.
The sealant of the '567 patent may also include, if desired, plasticizers, freeze-thaw stabilizers, colorants or pigments, pigment dispersants, anti-bacterial and/or anti-fungal preservatives, and agents for improving rheological properties such as thickeners and anti-slump agents. The plasticizer would generally be used in an amount of up to about 25% by weight and the other additives when present, will total up to about 3% by weight, based on the total weight of the composition. A plasticizer may be desirable to reduce the tack of the copolymer so that the sealant can have the desired tack-free time of less than 72 hours. (Column 4, lines 1-14)
Example 1 of the '567 patent discloses an acrylic copolymer latex sealant containing fumed silica and mineral oil as a plasticizer. (Mineral oil is typically added to sealant compositions as a tooling aid.) The acrylic copolymer latex (58% solids) is identified as containing 96% of a copolymer of ethyl acrylate and ethylhexyl acrylate, 2% acrylic acid and 2% N-methylolacrylamide and having a Tg of -27.degree. C. There is no disclosure with regard to the ratio of the ethyl acrylate and ethylhexyl acrylate nor of the stabilizing system used to prepare the latex.